Fuel cell shutdown and startup using a cathode recycle loop

ABSTRACT

A method and device for operating a fuel cell system. A recirculation loop coupled to a fuel cell cathode ensures that fluids passing through the cathode are recycled, thereby enabling reaction between residual oxygen in the recycled fluid and fuel that has been introduced into the recirculation loop until substantially all of the oxygen is reacted, leaving a substantially oxygen-free, predominantly nitrogen compound in the cathode and related flowpath. Thereafter, this compound can be redirected to purge the remaining residual hydrogen resident in the fuel cell&#39;s anode and related flowpath. While the present invention is usable during any period of system operation, it is especially valuable for operational conditions associated with starting up and shutting down a fuel cell system to inhibit the formation of high voltage potentials that could otherwise damage fuel cell catalysts or catalysts supports.

BACKGROUND OF THE INVENTION

The present invention relates generally to operating a fuel cell system,and more particularly to starting up and shutting down a fuel cell insuch a way as to minimize oxidation of catalyst support material whilemaintaining system simplicity.

The use of catalysts to facilitate the electrochemical reaction betweenhydrogen and oxygen in fuel cells are well known. Typically, thecatalyst is in the form of a noble metal powder that is distributed on asupport that is itself a powder of larger carbon or carbon-basedparticles. This powder-based approach allows for a significant increasein surface area upon which the aforementioned reaction can take place.While such a configuration provides for an efficient, compact reactorthat by spreading the relatively expensive catalyst (such as platinum)over a large area results in significant improvements in power outputwith simultaneous reduction in raw material cost, its effectiveness canbe limited by certain modes of operation. For example, even when theneed for electric current produced in a fuel cell is reduced or ceases,the residual oxygen and hydrogen reactants continue to generate an opencircuit voltage (typically around 0.9V or higher) that can lead tocatalyst and catalyst support oxidation, thereby reducing fuel celllife. Of even greater concern is the presence of a hydrogen-airinterface on one of the fuel cell electrodes (such as the anode) whileair is present on the other electrode (such as the cathode), which canlead to potentials of between 1.4V and 1.8V being generated. Theseelevated potentials exacerbate the aforementioned corrosion of thecatalyst and catalyst support material. This situation can occur duringstartup (when air is being purged by hydrogen) and during shutdown (whenair is entrained into the anode as hydrogen is consumed by cross-over).The present inventors have observed that ooperational transients,particularly repeated system startup and shutdown, appear to shortenfuel cell life much faster than the comparable steady-state operationthat takes place between such transients.

One way to alleviate the problem of residual fuel and oxidant is toinject an inert gas to purge both the anode flowpath and the cathodeflowpath immediately upon cell shutdown. This could be accomplished by,for example, injecting onboard nitrogen into the anode and cathodeflowpaths. However, this is disadvantageous, especially for manyvehicle-based fuel cell systems, as the on-board use of a parasiticgaseous nitrogen supply would take up precious vehicle space otherwiseused for passenger, comfort or safety features. Another approach is tointroduce air into the anode flowpath so that the air can react with theresidual hydrogen. By recirculating this mixture, the hydrogen can beignited or catalytically reacted until virtually none remains. By thisapproach, no on-board nitrogen purge gas is required. However, thissystem is disadvantageous in that complex system componentry, includingadditional pumps coupled to intricate valve networks all tied togetherwith precision control mechanisms, is required. Accordingly, thereexists a need for a fuel cell system that can be started up and shutdown without having to resort to approaches that require significantincreases in weight, volume or complexity.

BRIEF SUMMARY OF THE INVENTION

These needs are met by the present invention, wherein a fuel cell systemand a method of operating the system in such a way as to avoid thedetrimental effects of operational transients on system components isdisclosed. In accordance with a first aspect of the present invention, amethod of operating a fuel cell system is disclosed. The fuel cellsystem includes at least one fuel cell made up of at least an anode,cathode and membrane disposed between the anode and cathode, in additionto an anode flowpath configured to couple the anode to a fuel source anda cathode flowpath configured to couple the cathode to an oxygen source.The aforementioned flowpaths may include support equipment necessary forthe flow of fluids in and around the fuel cell, including piping andrelated conduit. Valves, pumps and related componentry, while alsoforming part of the flowpath, may also be individually discussed to moreclearly identify their function within their respective flowpath. In thepresent system, a recirculation loop is formed in the cathode flowpathand in conjunction with connectivity between the cathode flowpath andanode flowpath provides a means for generating an inert gas without thehigh temperatures associated with undiluted, stoichiometric combustion.While one type of fuel cell that can benefit from the present inventionis a proton exchange membrane (PEM) fuel cell, it will be appreciated bythose skilled in the art that the use of other fuel cell configurationsis also within the purview of the present invention. The operation ofthe present system occurs by decoupling the anode from the fuel sourceso that the flow of fuel is cut off, recycling fluid disposed in thecathode flowpath through the recirculation loop, introducing fuel intothe recirculation loop so that it can be reacted with the recycled fluiduntil the recycled fluid becomes substantially oxygen-depleted, and thenintroducing the substantially oxygen-depleted fluid into the anodeflowpath such that any fluid previously in the anode flowpath issubstantially removed. The term “oxygen source” and its variants is tobe understood broadly, encompassing any device, container or environment(including the ambient environment) configured to provide oxygen or asignificant oxygen-bearing compound, mixture or the like.

Optionally, the method includes the step of fluidly coupling a pressuresource to at least one of the fuel and oxygen sources. Such a pressuresource (for example, an air compressor) may be used to pressurize fluidcontained within the recirculation loop. The recycling step may furtherinclude closing a cathode exit valve and opening a cathode flowpathrecycle valve, both valves disposed within the recirculation loop. Inone form, the step of introducing the substantially oxygen-depleted gasinto the anode flowpath can include opening a purge valve that fluidlycouples the cathode flowpath to the anode flowpath. Preferably (althoughnot necessarily) the purge valve is disposed between the cathode and thecathode exit valve. The step of introducing fuel into the recirculationloop may include adjusting a fuel inerting valve that fluidly couplesthe anode flowpath to the cathode flowpath. Along with theaforementioned purge valve, this valve provides a direct bridge betweenthe two flowpaths.

The system may define at least a first operational state where thesystem is generating electricity, a second operational state where thesystem is not generating electricity, and a third operational statetransiently between the first and second operational states. Suchtransient operation involves those periods of operation over whichchanges in system power output occur. Two times such operation is ofparticular concern to the present invention is during system startup andshutdown. As such, transient operation is distinguished over steadystate operation, where the system output is substantially constant. Inone mode of operation, the decoupling, recycling, reacting and bothintroducing steps make up the third operational state. An additionalstep includes filling the anode flowpath with fuel once thesubstantially oxygen-depleted fluid has substantially purged the anodeflowpath. In this case, the system will be ready for normal operation(such as that associated with the aforementioned first operationalstate). One way to affect this last step is to fluidly isolate the anodeflowpath from the cathode flowpath, and fluidly coupling the fuel sourceto the anode. For example, fluidly isolating the anode flowpath from thecathode flowpath comprises closing the previously discussed purge valve.The step of fluidly isolating the anode flowpath from the cathodeflowpath may be achieved by closing the fuel inerting valve previouslydiscussed. The step of fluidly coupling the fuel source to the anode canbe performed by opening the fuel supply valve disposed within the anodeflowpath. The flow of fuel can be adjusted until the system is operatingnormally in its first operational state. Another option includesbleeding fluid from the oxygen source into the anode during a periodprior to normal operation, thereby providing additional heating toassist the system to more rapidly achieve optimal operating temperatures(for example, between 60° C. and 80° C.) during startup in lowtemperature environments. This bleeding step can include opening thepurge valve. Similarly, fuel can be bled from the fuel source into thecathode during a comparable period, also to assist with low temperaturestarting. To achieve this, the fuel inerting valve is opened.

Another optional step involves regulating the amount of fuel beingintroduced into the cathode flowpath in order to maintain asubstantially stoichiometric ratio between the fuel and the oxygenpresent in the recirculating fluid for the duration of the purge step.For example, the amount of oxygen present in the recirculating fluid canbe sensed so that the fuel inerting valve can be adjusted by an amountnecessary to maintain the substantially stoichiometric ratio. Acontroller can be included in the system so that the response to thesensed oxygen level can be performed automatically, such as by automatedmanipulation of one or more of the aforementioned valves. Regarding thereactants, it is preferred that the fuel be hydrogen-rich, examples ofwhich are methanol, hydrogen, methane (such as from natural gas) andgasoline. In cases where the fuel from the fuel source is notsubstantially pure hydrogen, a fuel processing system (such as amethanol reformer or other such reactor known to those skilled in theart) can be used to supply substantially pure hydrogen to the fuel cell.A preferred source of oxygen is air. Preferably, the reacting step takesplace in either or both a combustor that is fluidly coupled to thecathode flowpath and a catalyst disposed on the cathode. An additionalstep can be cooling the products produced during the reacting step. Thiscan be achieved by placing a cooler between the combustor and the fuelcell. The step of introducing the substantially oxygen-depleted fluidinto the anode flowpath may include fluidly coupling the cathodeflowpath downstream of the cathode with an inlet location in the anode.Yet another additional step may include filling the anode flowpath withair once the previously resident fuel has been substantially removed.The step of filling the anode flowpath with air can be performed byclosing the fuel inerting valve and opening the purge valve. Inaddition, the step of decoupling the anode from the fuel source can beaccomplished by closing a fuel supply valve.

According to another aspect of the present invention, a method ofpreparing a fuel cell system for startup is disclosed. As before, thefuel cell system includes at least one fuel cell made up of at least ananode, cathode and membrane disposed between the anode and cathode, inaddition to an anode flowpath configured to couple the anode to a fuelsource and a cathode flowpath configured to couple the cathode to anoxygen source. In addition, it includes a plurality of valves configuredto establish fluid communication between the anode flowpath and thecathode flowpath. The steps involved in the present method includeintroducing fuel from the fuel source into the cathode flowpath,recycling fluid disposed in the cathode flowpath through therecirculation loop, introducing fuel into the recirculation loop,reacting the fuel with the recycled fluid until the recycled fluidbecomes substantially oxygen-depleted and introducing the substantiallyoxygen-depleted fluid into the anode flowpath such that any fluidpreviously resident therein is substantially purged therefrom.

Optionally, the step of introducing the substantially oxygen-depletedfluid comprises opening a (previously discussed) purge valve thatfluidly couples the anode flowpath to the cathode flowpath, andsubsequently opening a (previously discussed) fuel supply valve thatfluidly couples the fuel source to the anode. Additionally, fluid can bebled from the oxygen source into the anode to facilitate low temperaturestarting. The step of bleeding air into the anode can include openingthe purge valve disposed between the cathode flowpath and the anodeflowpath. Moreover, fuel can be bled from the fuel source into thecathode to facilitate low temperature starting. The bleeding fuel stepcan be achieved by opening a fuel inerting valve similar to thatpreviously discussed.

According to yet another aspect of the present invention, a method oftransiently operating a fuel cell system is disclosed. The system isconfigured to define at least a first operational state where the systemis generating electricity and a second operational state where thesystem is not generating electricity. Components within the systeminclude at least one fuel cell, an anode flowpath, a cathode flowpath,all as previously described, a pressure source coupled to the oxygensource, and a plurality of valves, some of which are at least one valvedisposed within the recirculation loop, a purge valve and a fuelinerting valve. Steps in this method include placing the system in oneof the first or second operational states, decoupling the anode from thefuel source, arranging at least one valve disposed in the recirculationloop so that the fluid pressurized by the pressure source can berecycled through the loop, arranging the fuel inerting valve such thatfuel can be introduced from the fuel source into the cathode flowpath,reacting the fuel with the recycled fluid until the recycled fluidbecomes substantially oxygen-depleted and opening the purge valve suchthat the substantially oxygen-depleted fluid is introduced into theanode flowpath, thereby substantially purging the anode flowpath.

According to still another aspect of the invention, a device comprisingat least one fuel cell comprising an anode, a cathode and a membranedisposed between the anode and cathode is disclosed. The device alsoincludes an anode flowpath, a cathode flowpath and a plurality of valvesconfigured to establish fluid communication between the anode flowpathand the cathode flowpath, all as previously described. The plurality ofvalves includes a fuel supply valve disposed between the fuel source andthe anode, at least one valve disposed in the recirculation loop toselectively allow recycling of fluid in the loop, a fuel inerting valveand a purge valve.

Optionally, the device further includes a pressure source coupled to atleast one of the fuel source and the oxygen source. In addition, the atleast one valve disposed in the recirculating loop can be a plurality ofvalves, including a cathode exit valve configured to selectively controlback-pressure in an exhaust line in the cathode flowpath and a cathodeflowpath recycle valve disposed between the oxygen source and thepressure source. As previously discussed, the pressure source can besupplied by an air compressor. A combustor may also be included topromote reaction between fuel and oxygen. Also as before, a cooler maybe fluidly coupled downstream of the combustor, while a catalyst may bedisposed on the cathode to promote reaction between fuel and oxygen. Acontroller configured to regulate the amount of fuel being introducedinto the cathode flowpath may additionally be included, where an oxygensensor can be additionally included such that the controller isconfigured to manipulate the plurality of valves in response to a signalsent from the oxygen sensor. The device may further comprise a powerconversion mechanism configured to take electricity generated by thefuel cell system and convert it to motive power, and may furthermoreinclude a vehicle configured to house the fuel cell system and the powerconversion mechanism. The vehicle (an example of which can be a car,truck, motorcycle, aircraft or watercraft) is movably responsive to themotive power generated in the power conversion mechanism.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of the present invention can be bestunderstood when read in conjunction with the following drawings, wherelike structure is indicated with like reference numerals and in which:

FIG. 1A shows a block diagram of a fuel cell system configured forvehicular application;

FIG. 1B shows a representative fuel cell from the system of FIG. 1A;

FIG. 1C shows an enlargement of the region between the anode andmembrane of the fuel cell of FIG. 1B, highlighting the placement of acatalyst on a support, where the catalyst is used to facilitate theionization of the fuel;

FIG. 2A shows a block diagram of a fuel cell system according to oneaspect of the present invention;

FIG. 2B shows a variation of the system of FIG. 2A; and 19) FIG. 3 showsa vehicle employing the fuel cell system of either FIG. 2A or FIG. 2B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1A, a block diagram highlights the majorcomponents of a mobile fuel cell system 1 according to the presentinvention. The system includes a fuel delivery system 100 (made up offuel source 100A and oxygen source 100B), fuel processing system 200,fuel cell 300, one or more energy storage devices 400, a drivetrain 500and one or more motive devices 600, shown notionally as a wheel. Whilethe present system 1 is shown for mobile (such as vehicular)applications, it will be appreciated by those skilled in the art thatthe use of the fuel cell 300 and its ancillary equipment is equallyapplicable to stationary applications. It will also be appreciated bythose skilled in the art that other fuel delivery and fuel processingsystems are available. For example, there could be, in addition to afuel source 100A and oxygen source 100B, a water source (not shown).Likewise, in some variants where substantially purified fuel is alreadyavailable, the fuel processing system 200 may not be required. Theenergy storage devices 400 can be in the form of one or more batteries,capacitors, electricity converters, or even a motor to convert theelectric current coming from the fuel cell 300 into mechanical powersuch as rotating shaft power that can be used to operate drivetrain 500and one or more motive devices 600. The fuel processing system 200 maybe incorporated to convert a raw fuel, such as methanol into hydrogen orhydrogen-rich fuel for use in fuel cell 300; otherwise, inconfigurations where the fuel source 100A is already supplyingsubstantially pure hydrogen, the fuel processing system 200 may not berequired. Fuel cell 300 includes an anode 310, cathode 330, and anelectrolyte layer 320 disposed between anode 310 and cathode 330.Although only a single fuel cell 300 is shown, it will be appreciated bythose skilled in the art that fuel cell system 1 (especially those forvehicular and related applications) may be made from a stack of suchcells serially connected.

Referring next to FIGS. 1B and 1C, the anode 310 includes an electrodesubstrate 312 and catalyst layer 314 connected to a flow channel 316.The cathode 330 includes an electrode substrate 332 and catalyst layer334 connected to a flow channel 336. Flow channels 316, 336 form thepart of an anode flowpath and cathode flowpath (both described below)that contact their respective anode and cathode. Preferably, theelectrode substrates 312, 332 are porous to allow diffusion of fuel andoxygen, as well as the flow of water that forms as a result of thefuel-oxygen reaction. The catalyst layer 314 is made up of a catalyst314A dispersed on the surface of a support 314B. The electrolyte layer320, shown presently in the form of a proton exchange membrane, isplaced between each of the anode 310 and cathode 330 to allow theionized hydrogen to flow from the anode 310 to the cathode 330 whileinhibiting the passage of electrical current therethrough. Fuel(typically in the form of gaseous hydrogen) passes through flow channel316, allowing the fuel to diffuse through electrode substrate 312 andcome in contact with the catalyst 314A, through which theelectrochemical oxidation of the hydrogen fuel proceeds by what isbelieved to be a dissociate adsorption reaction. This reaction isfacilitated by catalyst 314A, typically in the form of finely-dividedparticles of a noble metal (such as platinum) that are dispersed overthe surface of the support 314B, which is typically carbon-based. Thepositively-charged hydrogen ion (proton) produced at the anode 310 thenpasses through the electrolyte 320 to react with the negatively-chargedoxygen ions generated at the cathode 330. The flow of liberatedelectrons sets up a current through the load 400 such that a motor orrelated current-responsive device may be turned. Load 400, shown in theform of the previously-discussed energy storage device, completes anelectrical flowpath between the anode and cathode of fuel cell 300. Anadditional pump (not shown) can be included to remove from the electrodesubstrates 312, 332 water that would otherwise collect and possiblyblock the porous passageways.

Referring next to FIGS. 2A and 2B, block diagrams of variations on thepresent system, both configured to reduce the hydrogen-oxygen interfacein fuel cell 300, are shown. An anode flowpath 340 fluidly couples fuelsource 100A to the anode 310 through a fuel supply valve 342. Oxygensource 100B is fluidly coupled to the cathode flowpath 350 such thatoxygen can be flowed past cathode 330. As shown with particularity inthe figures, a recirculation loop 352 is placed in the cathode flowpath350 to recycle a purge fluid (such as a nitrogen-rich gas) formed by thereaction of the hydrogen and oxygen. In addition to promoting thegeneration of the purge fluid through the consumption of oxygen, therecirculation loop 352 promotes uniformity of voltages between variouscells. The recirculation loop 352 includes a pressure source 360,combustor 370, oxygen sensor 380 and cooler 390, all fluidly coupled tohelp pass various fluids repeatedly through the cathode 330 duringoperational transients so that specific species can be reduced throughappropriate catalyzing or combustion reaction. Although only a singlecooler 390 (which can be in the form of a heat exchanger) is depicted,it will be appreciated by those skilled in the art that additionalcoolers, as well as other locations for the cooler 390, may be used. Byrecycling the hydrogen, oxygen and generated purge fluid (collectively,the fluid being recirculated) several times through cooler 390, asmaller temperature rise is encountered, thus reducing thermal burdenson the system. A cathode exit valve 354 is disposed downstream of thecathode 330 to control the flow of fluid between the exhaust of cathode330 and the recirculation loop 352, while a cathode flowpath recyclevalve 356 allows selective introduction of the fluid being recirculatedupstream of the cathode 330. Preferably, the pressure source 360 is anair compressor. The oxygen source 100B need not be shut off duringoperational transients (such as during startup or shutdown), asadditional air tends to not flow into the recirculation loop 352 due tothe presence of a dead head by virtue of the cathode exit valve 354being closed. Fluid communication between the anode flowpath 340 and thecathode flowpath 350 is established though a fuel inerting valve 344 anda purge valve 346, which can be actuated independently or in tandem toachieve the desired fluid flow between the flowpaths 340, 350.

Another feature that can be incorporated into the present system is acombustor 370 (also known as a burner) placed in the recirculation loop352 so that excess fuel can be burned. While the catalytic reactionbetween hydrogen and oxygen continues to occur at the cathode 330 aslong as both reactants are present in the recirculation loop 352, theburning process enabled by combustor 370 can speed up the transientoperation of the system by more quickly consuming the fuel, as well asreduce the likelihood of cathode overheating. To further speed up thereaction, a plurality of combustors can be used rather than a singlecombustor. Preferably, both the combustor 370 and the catalytic reactionat cathode 330 would be used to combine the best attributes of speed andcompleteness of hydrogen removal. In an additional feature, thecombustor 370 could include catalytic elements disposed therein tofurther react the hydrogen with the oxygen in the air. In this case, thecombustor elements (not shown) could be catalytically coated andelectrically heated. In either configuration, the prompt and thoroughremoval of the reactable species is beneficial because it allows rapidstarting and minimizes the aforementioned excess shutdown energy levelsthat would otherwise be generated as a result of the hydrogen-airinterface formed on the anode. In such instances, without the system ofthe present invention, excessive voltage potentials can develop thatwill attack the support 314B.

The shutdown sequence of fuel cell system 1 preferably starts withclosing fuel supply valve 342 to halt the flow of fuel to the anode 310.Next, the cathode exit valve 354 is closed while the cathode flowpathrecycle valve 356 is opened to force the fluid exiting the cathode 330into recirculation loop 352. In addition, the pressure source 360 (suchas an air compressor) is operated to promote the fluid flow through therecirculation loop 352, although it will be appreciated that if fluid isalready sufficiently pressurized, such additional pressure might not beneeded. The recirculation loop 352 is needed to move the fluid thoughthe combustor 370 or cathode 330 so that the air and fuel are mixed andreacted on the appropriate catalyst. Fuel inerting valve 344 can beadjusted during the period of fluid recirculation to allow theintroduction of hydrogen to the recirculating fluid, thereby reactingwith any oxygen still present therein. Oxygen sensor 380 can be used totrack the oxygen still present in the recirculation loop 352. The oxygensensor 380 can be used to maintain a stoichiometric ratio betweenhydrogen and oxygen in situations where continued purging, discussedbelow, is necessary. As shown with particularity in FIG. 2A, thereaction can occur on a catalyst in combustor 370, after which excessheat generated in the fluid by the combustion process can be reducedprior to introduction into the cathode 330 by passing the fluid througha heat exchanger (in the form of a cooler 390). Such a cooler could bedual-use, in that it may also be used to cool the air exiting the aircompressor. An alternate configuration, depicted in FIG. 2B, allows thereaction to take place on a catalyst on cathode 330. This variant wouldembody a simpler arrangement of components, possibly eliminating theneed for separate combustor and associated cooling mechanism. Dependingon the speed of reaction required, one or both of the configurations incombination could be used. In either of the above configurations, oncethe oxygen in the recirculating fluid is consumed, purge valve 346 isopened to allow fluid communication between the anode flowpath 340 andcathode flowpath 350. This allows the (now substantially oxygen-free)fluid that hitherto this time had been entrained in the recirculationloop 352 to purge the anode 310 of residual fuel and other fluids. Incases where the fluid being used to provide oxygen to the cathode isair, it will be appreciated that once the oxygen is substantiallyremoved, the remaining fluid will almost exclusively contain nitrogenwith traces of other gases. Since the nitrogen is inert, its presenceensures a suitable benign fluid for purging the anode and cathode. Inaddition, the nitrogen itself can be easily purged as needed.Preferably, the purge fluid is withdrawn from exit of cathode 330 totake advantage of the entire volume of the cathode 330 for purging.While it is generally the case that the fluid capacity of the cathodewithin a fuel cell is greater than that of the anode, there could becircumstances where additional purge fluid is required for the anode. Insuch cases, the flow of fuel into the cathode loop 350 through fuelinerting valve 344 could be adjusted so that a substantiallystoichiometric ratio between the fuel and oxygen in the cathode flowpath350 can be maintained. A feedback-based controller (not shown) can beincluded, and based on signals transmitted by oxygen sensor 380, can beused to keep the desired fuel-to-oxygen ratio in the fluid passingthrough the recirculation loop 352. Once the hydrogen has been purgedfrom the anode 310, it may then be purged with air (or other fluids, ifdesired). In this situation, the fuel inerting valve 344 is closed,allowing fluid in the recirculation loop 352 that has been pressurizedby pressure source 360 to flow into the anode 310 through purge valve346. This last step ensures that air is present on both the anode 310and cathode 330 during periods of inactivity of fuel cell 300.

The startup sequence of fuel cell system 1 would involve closing cathodeexit valve 354 while opening the cathode flowpath recycle valve 356 toforce the fluid exiting the cathode 330 into recirculation loop 352. Aswith the aforementioned shutdown sequence, the pressure source 360 isoperated to promote the fluid flow through the recirculation loop 352,if needed. Fuel inerting valve 344 can be adjusted during the period offluid recirculation to allow the introduction of hydrogen to therecirculating fluid, thereby reacting with any oxygen still presenttherein. As before, the reactions can take place in the devices ofeither of the embodiments shown in FIGS. 2A and 2B on a catalyst incombustor 370, on the cathode 330, or both. Purge valve 346 is opened toallow fluid communication between the anode flowpath 340 and cathodeflowpath 350. This allows the (now substantially oxygen-free) fluid thathitherto this time had been entrained in the recirculation loop 352 topurge the anode 310 of residual air and other fluids. Preferably, thepurge fluid is withdrawn from the exit of cathode 330 to take advantageof the entire volume of the cathode 330 for purging. Also as before, theflow of fuel into the cathode loop 350 through fuel inerting valve 344could be adjusted so that a substantially stoichiometric ratio betweenthe fuel and oxygen in the cathode flowpath 350 can be maintained. Afeedback-based controller (not shown) can be included, and based onsignals transmitted by oxygen sensor 380, can be used to keep thedesired fuel-to-oxygen ratio in the fluid passing through therecirculation loop 352. Once the oxygen has been purged from the anode310, the anode 310 may then be filled with hydrogen to begin normaloperation. At this time (if not before), the fuel inerting valve 344 andthe purge valve 346 would be closed, while the fuel supply valve 342would be opened. During normal operation, the flow of fuel can beadjusted in a manner similar to that discussed above, including the useof a controller. Air can be bled into the anode 310 by opening the purgevalve 346. Similarly, hydrogen can be bled into the cathode 330 byopening fuel inerting valve 344, thus providing additional heating toassist startup when the fuel cell 300 is exposed to low temperatureenvironments.

Referring next to FIG. 3 in conjunction with FIG. 1, a vehicleincorporating a fuel cell system according to the present invention isshown. Fuel cell 300 is fluidly coupled to a fuel supply 100A. While thevehicle is shown notionally as a car, it will be appreciated by thoseskilled in the art that the use of fuel cell systems in other vehicularforms is also within the scope of the present invention.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes may be made without departingfrom the scope of the invention, which is defined in the appendedclaims.

1. A method of operating a fuel cell system, said method comprising:configuring said system to include: at least one fuel cell comprising ananode, a cathode and a membrane disposed between said anode and cathode;an anode flowpath configured to couple said anode to a fuel source; anda cathode flowpath configured to couple said cathode to an oxygensource, said cathode flowpath including a recirculation loop disposedtherein; decoupling said anode from said fuel source; recycling fluiddisposed in said cathode flowpath through said recirculation loop;introducing fuel into said recirculation loop; reacting said fuel withsaid recycled fluid until said recycled fluid becomes substantiallyoxygen-depleted; and introducing said substantially oxygen-depletedfluid into said anode flowpath such that any fluid previously residenttherein is substantially purged therefrom.
 2. The method according toclaim 1, wherein said step of configuring said system comprises theadditional step of fluidly coupling a pressure source to at least one ofsaid fuel source and said oxygen source.
 3. The method according toclaim 2, comprising the additional step of pressurizing fluid containedwithin said recirculation loop.
 4. The method according to claim 1,wherein said recycling step further comprises closing a cathode exitvalve disposed within said recirculation loop.
 5. The method accordingto claim 4, wherein said recycling step further comprises opening acathode flowpath recycle valve disposed within said recirculation loop.6. The method according to claim 1, wherein said step of introducingsaid substantially oxygen-depleted gas into said anode flowpathcomprises opening a purge valve that fluidly couples said cathodeflowpath to said anode flowpath.
 7. The method according to claim 6,wherein said purge valve is disposed between said cathode and a cathodeexit valve.
 8. The method according to claim 1, wherein said step ofintroducing fuel into said recirculation loop comprises adjusting a fuelinerting valve that fluidly couples said anode flowpath to said cathodeflowpath.
 9. The method according to claim 1, wherein said systemdefines at least a first operational state where said system isgenerating electricity, a second operational state where said system isnot generating electricity, and a third operational state transientlybetween said first and second operational states.
 10. The methodaccording to claim 9, wherein said decoupling, recycling, reacting andboth introducing steps comprise said third operational state.
 11. Themethod according to claim 9, further comprising the step of filling saidanode flowpath with fuel once said substantially oxygen-depleted fluidhas substantially purged said anode flowpath.
 12. The method accordingto claim II, wherein said step of filling said anode flowpath with fuelonce said substantially oxygen-depleted fluid has substantially purgedsaid anode flowpath comprises fluidly isolating said anode flowpath fromsaid cathode flowpath, and fluidly coupling said fuel source to saidanode.
 13. The method according to claim 12, wherein said step offluidly isolating said anode flowpath from said cathode flowpathcomprises closing a purge valve disposed therebetween.
 14. The methodaccording to claim 12, wherein said step of fluidly isolating said anodeflowpath from said cathode flowpath further comprises closing a fuelinerting valve disposed therebetween.
 15. The method according to claim12, wherein said step of fluidly coupling said fuel source to said anodecomprises opening a fuel supply valve disposed within said anodeflowpath.
 16. The method according to claim 11, comprising theadditional step of placing said system in said first operational state.17. The method according to claim 16, comprising the additional step ofadjusting flow of said fuel until steady state operation is achieved.18. The method according to claim 11, comprising the additional step ofbleeding fluid from said oxygen source into said anode to assist saidfirst operational state.
 19. The method according to claim 18, whereinsaid bleeding step comprises opening a purge valve that fluidly couplessaid cathode flowpath to said anode flowpath.
 20. The method accordingto claim 11, comprising the additional step of bleeding fuel from saidfuel source into said cathode to assist said first operational state.21. The method according to claim 20, wherein said step of bleeding fuelinto said cathode comprises opening a fuel inerting valve that fluidlycouples said anode flowpath and said cathode flowpath.
 22. The methodaccording to claim 1, comprising the additional step of regulating theamount of fuel being introduced into said cathode flowpath in order tomaintain a substantially stoichiometric ratio between said fuel and saidoxygen present in said recirculating fluid at least until said oxygen issubstantially consumed in said reacting step.
 23. The method accordingto claim 22, wherein said step of regulating the amount of fuelcomprises: sensing the amount of oxygen present in said recirculatingfluid; and adjusting a fuel inerting valve that fluidly couples saidanode flowpath to said cathode flowpath by an amount necessary tomaintain said substantially stoichiometric ratio.
 24. The methodaccording to claim 1, wherein said fuel is hydrogen-rich.
 25. The methodaccording to claim 24, wherein said fuel is selected from the groupconsisting of methanol, hydrogen, methane and gasoline.
 26. The methodaccording to claim 1, wherein said oxygen source comprises air.
 27. Themethod according to claim 1, wherein said reacting step takes place in acombustor that is fluidly coupled to said cathode flowpath.
 28. Themethod according to claim 27, comprising the additional step of coolingproducts produced during said reacting step.
 29. The method according toclaim 28, comprising the additional step of disposing a cooler betweensaid combustor and said at least one fuel cell to effect said coolingstep.
 30. The method according to claim 1, wherein said reacting steptakes place on a catalyst disposed on said cathode.
 31. The methodaccording to claim 1, wherein said step of introducing saidsubstantially oxygen-depleted fluid into said anode flowpath comprisesfluidly coupling said cathode flowpath downstream of said cathode withan inlet location in said anode.
 32. The method according to claim 1,comprising the additional step of filling said anode flowpath with aironce said previously resident fuel has been substantially purgedtherefrom.
 33. The method according to claim 32, wherein said step offilling said anode flowpath with air is effected by closing a fuelinerting valve and opening a purge valve, each of said valves disposedbetween said anode flowpath and said cathode flowpath.
 34. The methodaccording to claim 1, wherein said step of decoupling said anode fromsaid fuel source is accomplished by closing a fuel supply valve.
 35. Amethod of preparing a fuel cell system for startup, said methodcomprising: configuring said system to comprise: at least one fuel cellcomprising an anode, a cathode and a membrane disposed between saidanode and cathode; an anode flowpath configured to couple said anode toa fuel source; a cathode flowpath configured to couple said cathode toan oxygen source, said cathode flowpath including a recirculation loopdisposed therein; and a plurality of valves configured to establishfluid communication between said anode flowpath and said cathodeflowpath; introducing fuel from said fuel source into said cathodeflowpath; recycling fluid disposed in said cathode flowpath through saidrecirculation loop; introducing fuel into said recirculation loop;reacting said fuel with said recycled fluid until said recycled fluidbecomes substantially oxygen-depleted; and introducing saidsubstantially oxygen-depleted fluid into said anode flowpath such thatany fluid previously resident therein is substantially purged therefrom.36. The method according to claim 35, wherein said step of introducingsaid substantially oxygen-depleted fluid comprises opening a purge valvethat fluidly couples said anode flowpath to said cathode flowpath, andsubsequently opening a fuel supply valve that fluidly couples said fuelsource to said anode.
 37. The method according to claim 36, comprisingthe additional step of bleeding fluid from said oxygen source into saidanode to facilitate low temperature starting.
 38. The method accordingto claim 37, wherein said step of bleeding air into said anode comprisesopening a purge valve disposed between said cathode flowpath and saidanode flowpath.
 39. The method according to claim 36, comprising theadditional step of bleeding fuel from said fuel source into said cathodeto facilitate low temperature starting.
 40. The method according toclaim 39, wherein said bleeding fuel step comprises opening a fuelinerting valve that fluidly couples said anode flowpath to said cathodeflowpath.
 41. A method of transiently operating a fuel cell system, saidmethod comprising: configuring said system to define at least a firstoperational state where said system is generating electricity and asecond operational state where said system is not generatingelectricity, said system comprising: at least one fuel cell comprisingan anode, a cathode and a membrane disposed between said anode andcathode; an anode flowpath configured to couple said anode to a fuelsource; a cathode flowpath configured to couple said cathode to anoxygen source, said cathode flowpath including a recirculation loopdisposed therein; at least one valve disposed within said recirculationloop to selectively allow recirculation of fluid therethrough; a purgevalve that fluidly couples said cathode flowpath to said anode flowpath;a fuel inerting valve that fluidly couples said anode flowpath to saidcathode flowpath; and a pressure source coupled to said oxygen source;placing said system in one of said first or second operational states;decoupling said anode from said fuel source; arranging said at least onevalve disposed in said recirculation loop such that said fluidpressurized by said pressure source can be recycled therethrough;arranging said fuel inerting valve such that fuel can be introduced fromsaid fuel source into said cathode flowpath; reacting said fuel withsaid recycled fluid until said recycled fluid becomes substantiallyoxygen-depleted; and opening said purge valve such that saidsubstantially oxygen-depleted fluid is introduced into said anodeflowpath, thereby substantially purging said anode flowpath.
 42. Adevice comprising: at least one fuel cell comprising an anode, a cathodeand a membrane disposed between said anode and cathode; an anodeflowpath configured to couple said anode to a fuel source; a cathodeflowpath configured to couple said cathode to an oxygen source, saidcathode flowpath including a recirculation loop disposed therein; and aplurality of valves configured to establish fluid communication betweensaid anode flowpath and said cathode flowpath, said plurality of valvescomprising: a fuel supply valve disposed between said fuel source andsaid anode; at least one valve disposed in said recirculation loop toselectively allow recycling of fluid therethrough; a fuel inerting valvedisposed between said anode flowpath and said cathode flowpath to allowselective fluid communication therebetween; and a purge valve disposedbetween said anode flowpath and said cathode flowpath to allow selectivefluid communication therebetween.
 43. The device according to claim 42,further comprising a pressure source coupled to at least one of saidfuel source and said oxygen source.
 44. The device according to claim43, wherein said at least one valve disposed in said recirculating loopcomprises: a cathode exit valve configured to selectively controlback-pressure in an exhaust line in said cathode flowpath; and a cathodeflowpath recycle valve disposed between said oxygen source and saidpressure source.
 45. The device according to claim 43, wherein saidpressure source comprises an air compressor.
 46. The device according toclaim 42, further comprising a combustor configured to promote reactionbetween fuel and oxygen.
 47. The device according to claim 46, furthercomprising a cooler fluidly coupled downstream of said combustor. 48.The device according to claim 42, further comprising a catalyst disposedon said cathode.
 49. The device according to claim 42, furthercomprising a controller configured to regulate the amount of fuel beingintroduced into said cathode flowpath.
 50. The device according to claim49, further comprising an oxygen sensor such that said controller isconfigured to manipulate said plurality of valves in response to asignal sent from said oxygen sensor.
 51. A device according to claim 42,wherein said device further comprises a power conversion mechanismconfigured to take electricity generated by said fuel cell system andconvert it to motive power.
 52. A device according to claim 51, whereinsaid device further comprises a vehicle configured to house said fuelcell system and said power conversion mechanism, said vehicle movablyresponsive to said motive power generated in said power conversionmechanism.